Synopsis Approximately one hour after take-off, the HS 748 aircraft suffered a loss of main hydraulic system pressure. The crew diverted the aircraft to the company's main base in Thompson, Manitoba. During the landing roll, the aircraft departed the right side of the runway. The passengers were deplaned through the aft main cabin door; the evacuation went smoothly and there were no injuries. The Board determined that main hydraulic pressure was lost due to a leak in the right maxaret (anti-skid) return line, which failed due to overdeflection from an undetermined source. Directional control was lost during the landing roll due to a loss of brake pressure, the cause of which was not determined. As the aircraft slowed down, it weathercocked into the prevailing cross-wind and departed the runway. Ce rapport est galement disponible en franais. 1.0 Factual Information 1.1 History of the Flight At 1341 central standard time (CST)(1) on 20 January 1994, a Calm Air International Hawker Siddeley HS 748, operating as Canadian Partner flight CAV 1557, departed the Winnipeg International Airport, Manitoba, on a scheduled domestic flight to Thompson, with one scheduled en route stop at Flin Flon. Prior to departure, the crew conducted a pre-flight check of the aircraft which included a check of the hydraulic system fluid quantity. Approximately one hour after departure, at about 150 nautical miles (nm)(2) north of Winnipeg, the right hydraulic pump low-flow warning light began to flicker. This was soon followed by a flicker of the left hydraulic pump low-flow warning light and a loss of main hydraulic pressure, which soon dropped below the normal operating range of 2,000 to 2,500 pounds per square inch (psi) to a pressure of 1,800 psi; both low-flow warning lights then remained ON constantly. The crew radioed the company duty manager, and a decision was made to divert from the planned en route stop in Flin Flon and proceed to the company's main base in Thompson. The cabin crew and passengers were advised that the aircraft was experiencing hydraulic problems, and that they would be proceeding directly to Thompson. Approximately 25 minutes prior to the aircraft's arrival in Thompson, the main hydraulic system pressure dropped to zero. The crew consulted the checklist, carried out the hydraulic emergency drill, and turned the nosewheel steering off. The brake system pressure gauge remained steady at about 2,000 psi, which indicated to the crew that there should be sufficient stored back-up brake pressure available to stop the aircraft. As a precaution, however, the crew briefed the flight attendants on the situation and asked them to prepare the cabin for evacuation in case the aircraft did not remain on the runway. The crew advised the Thompson Flight Service Station (FSS) that they were having hydraulic problems and requested that the airport Emergency Response Services (ERS) equipment be put on standby. On arrival in Thompson, the crew received an update on the weather and runway surface conditions from Thompson tower, and were instructed to land on runway 23. The crew lowered the landing gear manually, using the auxiliary hydraulic system, and had to circle the airport several times before obtaining a down-and-locked indication for the landing gear. Once the crew received confirmation that the passenger cabin was secure, they proceeded to land. Following a normal touchdown, the crew pulled the propeller flight fine pitch stops to slow the aircraft, while maintaining directional control using the rudder. As the aircraft decelerated, the pilot slowly applied the wheel brakes, but felt no brake pressure or braking action. The first officer looked at the brake pressure gauge and observed it fall rapidly to zero. With the aircraft slowing, the rudder became less effective and the aircraft began to veer to the right. The passengers were issued the brace command and the crew immediately feathered the propellers as the aircraft departed the right side of the runway. The aircraft slowed gradually in the deep snow and travelled approximately 40 feet before coming to a stop. The passengers were deplaned through the aft main cabin door; there were no injuries and the evacuation went smoothly. The occurrence happened at the Thompson Airport at latitude 5548'N and longitude 09752'W at approximately 1615 CST, during the hours of daylight, at an elevation of 729 feet above sea level (asl). 1.2 Injuries to Persons 1.3 Damage to Aircraft The right propeller sustained minor propeller blade deformation from contact with the snow. The right propeller was sent for repair and the right engine was removed for inspection. 1.4 Other Damage 1.5 Personnel Information 1.6 Aircraft Information A review of the aircraft's technical records and discussions with company maintenance personnel revealed that this aircraft, as well as the other company HS 748 aircraft, had a history of hydraulic problems associated with cold weather operations. It was found that, during extreme cold temperatures, small hydraulic system leaks or seeps occurred as the hydraulic seals lost their effectiveness. It is also not uncommon to have the hydraulic low-flow lights flicker after start until the system fluid has been cycled. During the 30 days preceding the occurrence, the following hydraulic system snags were recorded on this particular aircraft: on 30 December 1993, the No. 2 maxaret anti-skid unit was replaced; on 11 January 1994, the port engine low-flow light was reported to be ON continuously; the problem was rectified by bleeding both engine-driven hydraulic pumps and the emergency reservoir; on 12 January 1994, both low-flow lights were reported to be ON continuously; the port hydraulic pump was replaced, and both engine-driven hydraulic pumps and the emergency reservoir were bled. On the day of the occurrence, 1 litres of hydraulic fluid were added to the aircraft to bring the hydraulic reservoir up to normal operating capacity. 1.7 Meteorological Information The Environment Canada weather observation for Thompson was issued at 2200 UTC, and estimated cloud layers at 1,200 feet agl broken and 21,000 feet broken with a visibility of 10 miles in very light snow and blowing snow. The temperature was minus 17 degrees Celsius with the winds from 270 degrees at 17 knots. The Thompson tower weather observation at the time of the occurrence included observed surface winds from 270 to 300 degrees at 10 knots gusting to 15. 1.8 Aerodrome Information The Thompson Airport is operated by the local government District of Mystery Lake, and has an operational FSS and control tower. Runway 05/23, the main runway, is asphalt-covered and is 5,800 feet long by 150 feet wide. Midpoint access to the runway is provided by taxiway Alpha and taxiway Charlie. The aircraft departed the right side of the runway just beyond taxiway Charlie or approximately 2,450 feet beyond the threshold of runway 23. A runway surface condition report was issued by Thompson tower at 2130 UTC. The runway condition was reported as 90 per cent frost covered with 10 per cent compacted snow patches. The James Brake Index (JBI) for runway 23 was reported at 0.420. The Aeronautical Information Publication(AIP) indicates that a JBI of 0.420 could increase the stopping distance by as much as 45 per cent. 1.9 Aircraft Hydraulic Power The aircraft hydraulic power system consists of a main system and an auxiliary system. The main system hydraulic fluid is supplied by two engine-driven pumps, and is stored under pressure in two main and two brake system accumulators. Hydraulic power is used for the operation of the landing gear, wheel brakes, nosewheel steering and propeller brake. The main system working pressure cycles between 2,000 and 2,500 psi. A flow indicator valve in each pump delivery line controls an associated amber warning light which illuminates if the pump fails or the flow rate falls below 0.8 gallons/min. The auxiliary system supplies hydraulic pressure via a double-acting hand pump using fluid drawn from an auxiliary reservoir. This auxiliary hydraulic system is primarily used for lowering the landing gear following a main system hydraulic failure. It can also be used, however, to provide hydraulic pressure for ground servicing or to build up pressure in the main and brake system accumulators without running the engines. 1.10 Wheel Brakes There are two independent wheel brake pressure systems. One system supplies hydraulic pressure to the inboard brake on each main landing gear and the other to the outboard brake. Each wheel is equipped with a maxaret (anti-skid) unit designed to permit maximum braking action by momentarily bleeding off brake pressure to prevent wheel lockup; hydraulic fluid bled during maxaret anti-skid unit activation returns to the main hydraulic system reservoir. Non-return valves (NRVs) are used to isolate the brake system from the main hydraulic system. In the event of a main system failure, hydraulic fluid stored in the brake accumulators will allow braking during landing. The accumulators are fully charged at 2,500 psi and are considered to be discharged at 1,300 psi. The flight manual states that, with a full charge, approximately nine moderate brake applications are available. 1.11 Post Occurrence Investigation 1.11.1 Aircraft Hydraulic System Examination After the occurrence, the aircraft was brought into a warm hangar to assess the damage and isolate the cause of the hydraulic system failure. Fluid was added to the main system hydraulic reservoir and a normal system nitrogen pre-charge was added. Fluid leakage was observed in the right main landing gear maxaret return line (P/N 200140651), located along and just behind the main landing gear strut. 1.11.2 Maxaret Return Line Examination The line was removed from the aircraft and sent to the TSB Engineering Branch Laboratory for examination (report number LP 21/94). The line was determined to be an Aeroquip type 666-5 medium-pressure hose with a spiral extruded Teflon resin inner tube and a Type 300 Series stainless steel wire braid outer cover. The steel braid was cut and removed in the area of the leak, and a split, encompassing approximately one-third of the tube circumference, was found at the leak location. The orientation of the circumferential split, relative to the end fitting, was at 90 degrees to the normal flexing direction of the installed line, and strongly suggests that the failure was the result of an overdeflection by some external loading rather than normal service flexing. The line was slightly stiffer than a sample of a more recently manufactured line; however, the reduced flexibility of the line was not assessed to be the primary cause of the failure. A review of the aircraft technical records indicated that the line was most likely installed on 18 March 1988 at an airframe time of 10,661.9 hours (11,050 landings) and the failure occurred at 24,788.2 hours (31,031 landings). The line has no specified service limit life. 1.11.3 Brake System Check The maxaret line was replaced and the hydraulic system was brought up to a normal operating pressure and capacity. Other than a few minor drips, no additional system leaks could be found. Main and auxiliary system checks were carried out with no faults found. When a brake system check was carried out, the no. 3 (right, inboard) maxaret unit failed the wheel spin-up test. The maxaret unit was replaced and the hydraulic system was re-bled. No further system faults were found. 1.11.4 Maxaret Unit Examination and Testing The no. 3 maxaret unit was bench-tested and found to operate normally. Disassembly of the unit revealed that the internal clutch mechanism was contaminated with grease. However, this contamination would not likely have resulted in the early release of brake system pressure reported by the crew, but just the opposite, because slippage of the grease-contaminated clutch would have caused a delay in the operation of the maxaret unit and the bleeding of the brake system pressure. 1.12 Decision to Divert Following the main hydraulic system failure, the crew considered returning to Winnipeg, but the aircraft landing weight was slightly high. They also considered landing at The Pas before the hydraulic pressure decreased further; however there was no equipment available to tow them off the runway. Although there was a cross-wind in Thompson and the longer flight duration meant a possible continued decrease in the hydraulic pressure, there was equipment and maintenance available at Thompson; therefore, the crew decided to continue. 1.13 Cabin Preparation and Evacuation When the main hydraulic system pressure fell to zero, the flight crew advised the flight attendants to prepare the cabin for an emergency landing. The flight attendants briefed the passengers on the emergency procedures for the HS 748 aircraft and then instructed them on how to assume the brace position upon hearing the voice command Brace. They then informed the passengers that, if the aircraft were to leave the runway, they would likely be deplaning through the rear cabin doors. The passengers were asked to put on their winter coats, and two passengers were asked to assist in the evacuation, if it became ecessary. After touchdown, when it became apparent that the aircraft was going to depart the runway, the first officer issued the Brace command and the flight attendants repeated the instruction. As the aircraft came to a stop, the flight attendants heard the engines spool down and asked the passengers to release their seat-belts and follow them to the back of the aircraft. The flight attendants opened the aft main cabin door and noticed that the snow was only two to three feet from the door sill of the aircraft. They instructed the two volunteers to lower themselves to the ground to assist the passengers off. While the evacuation was in progress, the first officer deplaned through the forward freight door. He went to the back of the aircraft and advised the flight attendants that it would be safe to lower the folding aircraft stairs. The remaining passengers deplaned using the stairs and, once off the aircraft, they were directed to walk the short distance to the airport terminal building. The evacuation went smoothly and no passengers were injured.